1. Field of the Invention
This invention relates to a catalyst for catalytic combustion of a fuel used in an apparatus wherein high-temperature gases formed from said fuel are utilized, and particularly to a catalyst for catalytic combustion comprising a precious metal supported on a special carrier, said catalyst being very slight in lowering in performance even under high temperatures.
2. Description of the Prior Art
The catalytic combustion method wherein the reaction between a fuel and oxygen is accelerated on a catalyst is particularly noticed recently and has the following features: (1) complete combustion is possible at low temperatures, (2) complete combustion is possible in a wide fuel/air ratio, and (3) thermal NO.sub.x (x=1, 2, etc.) is hardly produced. As techniques applying the catalytic combustion technique, there are (a) an oxidation treatment of organic solvents and bad odors, (b) an automobile exhaust gas treatment, (c) a catalytic combustion type heater, etc. Further, techniques applying the catalytic combustion technique so as to make the best use of the above-mentioned features have been developed in various fields such as large-capacity boilers, gas turbines, jet engines for air planes, etc. These are techniques applying high-temperature gases obtained by catalytic combustion of a fuel and are essentially different from the automobile exhaust gas treatment and the process for removing harmful components such as removing a bad odor.
The temperature at which the catalyst for producing high-temperature gases by catalytic combustion of a fuel is used is usually 400.degree. C. or higher, and sometimes reaches 1400.degree. to 1500.degree. C. Therefore, the catalyst is required to have catalytic activity in a temperature range of from 400.degree. C. to 1400.degree. C. or higher and at the same time to be slight in deterioration by heat particularly at high temperatures. The temperatures used in conventional catalytic combustion methods are 200.degree. to 500.degree. C. for removing a bad odor and 800.degree. to 900.degree. C. at most for automobile exhaust gas treatment.
As catalysts, there are generally known those carrying precious metals, particularly platinum, palladium and rhodium on carriers having a large specific surface area such as alumina, silica, etc. (e.g. disclosed in U.S. Pat. No. 4,220,559). Since these precious metal catalysts are thermally stable compared with other metal oxide catalysts, they are used under the temperature conditions of 800.degree. to 900.degree. C. such as for automobile exhaust gas treatment. But in a process wherein heat energy is taken out of chemical energy by catalytic combustion of a fuel, a catalyst is used at a reaction temperature of 1000.degree. C. or higher, sometimes up to 1400.degree. C. to 1500.degree. C. depending on conditions. Under such conditions, even the precious metal catalysts are suffered from deterioration by heat to lower their performance remarkably. As causes for such deterioration, it can be pointed out that finely divided precious metal particles dispersed on carrier are agglomerated due to the high temperature or the specific surface area of carrier is reduced due to sintering of the carrier.